docs: def _get_dL2L(self, imt_per):
          """
          Table 3 and equation 19 of 2013 report.
          """
          if imt_per < 0.18:
              dL2L = -0.06
          elif 0.18 <= imt_per < 0.35:
              dL2L = self._interp_function(0.12, -0.06, 0.35, 0.18, imt_per)
          elif 0.35 <= imt_per <= 10:
              dL2L = self._interp_function(0.65, 0.12, 10, 0.35, imt_per)
          else:
              dL2L = 0
  
          return dL2L